Pseudomonas exotoxin kills most cells and tissues. However, polarized epithelial cells are resistant to Pseudomonas exotoxin and in some circumstances to diphtheria toxin. The basis for this finding is currently being investigated. Toxins may take a novel pathway and transcytose across polarized cells rather than kill them. cDNA microarrays are being used to compare gene expression profiles in toxin resistant cells with those of toxin sensitive cells. Preliminary data indicate that resistance cannot be explained on the basis that polarized cells lack the expression of any known member of the toxin intracellular pathway. Novel candidate genes are being pursued. The administration of Pseudomonas exotoxin onto the airway epithelia of mice was used as a model system to follow the fate of toxin released by Pseudomonas aeruginosa during infection of CF patients. Results indicate that the toxin can cross to the serosal side of the epithelium and destroy cells of the spleen and liver as well as those in local lymph nodes. The delivery of a non-toxic version of the toxin to local lymph nodes and spleen may be a useful path for vaccine administration. Neurokinin receptors can function as surface targets for neuropeptide-toxin conjugates and this may lead to selective nerve cell ablation. Substance P-PE35 conjugates were administered into the lumbar region of the spinal column of rats. Eight days later, sections of spinal column of treated Vs non-treated animals were compared. In rats treated with SubP-PE35 all neurons staining for the NK1 receptor had been ablated. Despite this, rats appeared to act no different from their untreated litter mates. We are investigating the toxin's ability to act as a carrier for key pilin sequences. The PE-pilin protein is being developed as a candidate vaccine for cystic fibrosis.